docs: def get_choices(self):
          """
          Return a list of choices for source files found in configured layout
          template directories.
          """
          choices = []
          for label_prefix, templates_dir, template_name_prefix in \
                  appsettings.LAYOUT_TEMPLATES:
              source_dir = os.path.join(templates_dir, template_name_prefix)
              # Walk directories, appending a choice for each source file.
              for local, dirs, files in os.walk(source_dir, followlinks=True):
                  for source_file in files:
                      template_name = os.path.join(
                          template_name_prefix, source_file)
                      label = '%s: %s' % (label_prefix, source_file)
                      choices.append((template_name, label))
          return choices